Semiconductor devices that can efficiently and sensitively detect optical signals are of obvious interest in a variety of technological fields, (e.g., in optical communications), particularly if the devices can have high bandwidth (e.g., in excess of 1 GHz or, preferably, several GHz). A novel fast, efficient semiconductor photodetector thus would be of interest.
Known semiconductor photodetectors typically have an (electrical) output that is substantially proportional to incident light intensity, frequently necessitating provision of circuitry to protect the (typically highly sensitive) input amplifier against overload. Availability of a photodetector whose output saturates at some convenient level could result in simpler and therefore less costly detection apparatus and thus would be of interest. Furthermore, known semiconductor high speed photodetectors typically have the characteristics of a current source, requiring the use of a load resistor to provide a voltage signal to the input amplifier. Availability of a photodetector substantially having the characteristics of a voltage source could result in simplified apparatus. This application discloses a novel device having these and other advantageous features.
F. Capasso et al. (Physical Review Letters, Vol. 25, pp. 2318-2321) described a transient electrical polarization phenomenon in sawtooth superlattices. The effect was observed in a p-doped graded-gap AlGaAs structure such as is schematically depicted in FIG. 1, wherein E.sub.C and E.sub.V refer to the conduction band edge and the valence band edge, respectively, and E.sub.F is the Fermi energy. The superlattice period d is also indicated. Band diagrams such as FIG. 1 are well known to those skilled in the art.
Sudden illumination of the structure of FIG. 1 with high energy (i.e., of energy greater than the minimum bandgap) photons was observed to give rise to a substantial transient voltage across the superlattice, with the voltage decaying rapidly, with a time constant of order .tau..sub.M, the Maxwell relaxation time. No steady-state polarization was observed under dc illumination.
The effect arises when the electron transit time .tau..sub.d across one superlattice period d is shorter than .tau..sub.M of the p-doped material. An abrupt light pulse generates electron-hole pairs and sets electrons in motion that temporarily upset the local balance of charge. Because of the sawtooth asymmetry of the superlattice, the polarization from each period adds up, analogously to a pyroelectric, developing a voltage across the superlattice. The rise time of the prior art transient polarization voltage is also of order .tau..sub.M.